1. Field of the Invention
This invention relates generally to an apparatus for spray coating discrete particles while such particles are suspended in a fluidized bed. More particularly, the present invention relates to a fluid-bed particle coater having a dual-jet and spray arrangement which is adapted to automatically monitor and control all essential processing parameters of the fluidized-bed coating operation.
2. Description of the Prior Art
The application of fluid-bed technology for coating tablets, granules, pellets and other discrete particles is well known in the prior art, having been described in considerable detail in such patent literature as U.S. Pat. Nos. 2,648,609, 2,799,241, 3,089,824, 3,196,827 and 3,207,824 to Wurster and Wurster et al. Such fluidized-bed coating processes find particular application in the pharmaceutical field and generally involve supporting uncoated particles in a vertical column by injecting a continuous stream of air from the bottom of a column. A coating solution is atomized and sprayed onto the air-suspended particles, and the particles are then dried while supported by the air. The drying time of the applied coating should be regulated by controlling the atomization rate and/or the temperature of the supporting air stream. Also, the velocity of the air stream should be adjustable so that the air-suspended particles are maintained in a relatively confined region of the column. Moreover, the air stream is usually directed into one portion of the suspended bed of particles at a higher velocity than in the remaining portion of the fluidized bed. This causes the particles to flow upwardly in the portion of the bed subjected to the higher velocity air and induces a downward flow in the remaining portion of the bed to create a cyclical vertical movement of the particles within a generally central region of the column. Since the particles are repeatedly recirculated within the column, successive layers of coating solution are applied to the particles until a coating of the desired thickness is formed.
The apparatus that has been commercially utilized for conducting a fluidized-bed coating procedure typically comprises a cylindrical column having a tapered lower section terminating in an air inlet opening. A screen or other porous member usually extends across the air inlet to retain particles in the column when the flow pressurized air is discontinued. At or above the center of the screen is an upwardly-directed nozzle for spraying liquid coating materials upwardly into the central zone of the column. As the air-suspended particles commence their upward travel within the central zone of the column, they are coated with the liquid coating discharged from the nozzle. The liquid deposited on the particles dries as these coated particles migrate upwardly within the central zone and as they travel downwardly through an annular outer zone of the column. The dried coated particles subsequently reenter the central region of the column where the coating is successively applied until a satisfactory film coat is obtained.
A variety of modifications to the general apparatus and procedure described hereinabove has been proposed in an attempt to improve the overall efficiency of the fluidized-bed coating operation. For instance, U.S. Pat. No. 3,241,520 to Wurster et al. discloses an apparatus wherein the central region of the column is provided with a partition for separating the upwardly and downwardly moving particle streams in order to establish a greater upward velocity of air in the coating zone. However, this as well as other prior art structural modifications fail to overcome the reported disadvantages of fluidized-bed coating systems. Such acknowledged disadvantages include surface imperfections resulting from vigorous agitation of the coated granules and loss of coating material to the air exhaust, which present certain fundamental problems to the pharmaceutical industry. Also, undesirable particle elutriation from the fluidized bed presents both a health and safety hazard, resulting in both the loss of scarce product materials and the need for expensive pollution control equipment. It was further found that these prior art coating procedures were imprecisely controlled by conventional equipment. Adjustments to these fluidized systems to account for variations in the processing conditions during operation were difficult to accomplish, causing variations in the finished products.